1. Field of the Invention
The present invention relates to a magnesium ion containing non-aqueous electrolyte showing excellent electrochemical characteristics, a production process thereof, as well as an electrochemical device using the electrolyte.
2. Description of the Related Art
Materials suitable to a negative electrode active substance as one of basic constituent materials of a battery include those metals that tend to emit electrons to form cations, that is, metals of high ionization tendency. Metal lithium is an example thereof. A battery using metal lithium as a negative electrode active substance is constituted as a battery using a non-aqueous electrolyte, in combination with various positive electrode active substances such as oxides or sulfides into products and used as a power source mainly for small-sized portable electronic equipments.
In recent years, for improving the conveniency, decrease in the size, weight reduction, decrease in the thickness, and improvement in the performance for the small-sized portable electronic equipments have been proceeded year by year steadily and, accordingly, a battery used as a power source for the equipments described above have been demanded to be small in the size, light in the weight, reduced in the thickness and, particularly, have high capacity. Accordingly, it can be considered that the battery is better as the capacitance per unit mass (mAh/g) or capacitance per unit volume (mAh/cm3) is higher for the negative electrode active substance and the positive electrode active substance constituting the battery.
When comparing the energy capacity per unit mass, the energy capacity of metal lithium (Li) is larger and more excellent compared with the energy capacity of other metals. Therefore, various studies have been reported so far on lithium secondary batteries. However, lithium secondary batteries involve a problem in view of the safety and, further, lithium is restricted in view of the resource and is expensive.
On the contrary, magnesium is abundant in view of the resource and less expensive compared with lithium. Further, metal magnesium has a large energy capacity per unit volume and has higher energy density compared with metal lithium. In addition, high safety can be expected in a case of use for batteries. As described above, the magnesium secondary battery is a secondary battery capable of compensating the drawbacks of the lithium secondary battery. In view of the above, an importance is attached at present to a development of a non-aqueous electrolyte battery using metal magnesium for the negative electrode active substance as the high capacity battery in the next generation. As in the examples described above, metal magnesium and magnesium ions are most promising materials as the electrolyte active substance respectively in electrochemical devices and charge carriers in electrolytes.
In the design for the electrochemical device using metal magnesium or magnesium ion, selection of the electrolyte is extremely important. For example, as the solvent for constituting the electrolyte, not only water and protonic organic solvent but also aprotonic organic solvents such as esters or acrylonitrile can not be used. This is because a passivation film not transmitting magnesium ions is formed on the surface of the metal magnesium when using them. The problem for the occurrence of the passivation film is one of bars for putting the magnesium secondary battery into practical use.
As the electrolyte free from the problem for the generation of the passivation film and capable of electrochemically utilizing magnesium, ether solutions of Grignard reagent (RMgX: R is alkyl group or aryl group, X is chlorine, bromine or iodine) have been known long since. When such electrolyte is used, metal magnesium can be precipitated and dissolved reversively. However, it involves a problem that the oxidation potential of the electrolyte is as low as about +1.5 V relative to an equilibrium potential of metal magnesium and the potential window is insufficient for use as an electrochemical device (refer to description in Non-Patent Document 1 (A. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich, E. Levi “Prototype systems for rechargeable magnesium batteries”. Nature, 407, p. 724-727 (2000) (pages 724 to 726, FIG. 3)).
On the contrary, in 2000, Aurbach, et al. in Bar-Iran University have found a tetrahydrofuran (THF) solution of Mg(ZRnX4-n)2 capable of electrochemically utilizing magnesium (where Z is boron or aluminum, R is a hydrocarbon group, X is a halogen, and n=0 to 3) (refer to Patent Document 1 (JP-T No. 2003-512704 (pages 12 to 19, FIG. 3)) and Non-Patent Document 1). They not only conducted precipitation and dissolution of metal magnesium but also trially manufactured magnesium secondary batteries enabling charge/discharge at 2000 cycles or more.
On the other hand, Patent Document 2, JP-A No. 2004-259650 (pages 4 and 5, FIG. 1) reported that the problem of low oxidation potential of the electrolyte could be solved by using an etheric solution of an aromatic Grignard reagent RMgX (X is chlorine or bromine) in which R is an acryl group.